When he's not acting or writing books, Steve Earle is recording albums like his latest with the Dukes and Duchesses, The Low Highway. He visited the Late Show With David Letterman last night to perform "Invisible," a song that built from a simple acoustic-guitar intro to include swells of pedal steel from Chris Masterson and mournful violin from Eleanor Masterson, his wife, while Earle sang yearning lyrics from the perspective of an overlooked population: the homeless and indigent.